(a) Field of the Invention
The present invention relates to a grinding apparatus and a grinding method for wafers, and more particularly, to a double-side grinding apparatus and a double-side grinding method by which hydrostatic supporting units are placed on either surface side of a semiconductor wafer, and a fluid is supplied to spaces between the wafer and the hydrostatic supporting units, so as to support the wafer physically in a noncontact manner, and simultaneously grind both surfaces of the wafer.
(b) Description of the Related Art
One type of double-side grinding apparatus for semiconductor wafers is an apparatus that hydrostatically supports either surface of a wafer through a fluid, and simultaneously grinds either surface by pressing grindstones against the wafer while rotating the wafer. In this apparatus, hydrostatic supporting units to apply static pressure to the wafer through a fluid are placed at very short distances from the wafer, and the fluid (such as water) is supplied to the very narrow spaces. As a result, the wafer is sandwiched by fluid layers, and is supported without any physical contact with other components. A static supporting unit is normally formed with a static pad member having pockets formed in its wafer facing surface. By supplying a fluid to the pockets, the wafer is hydrostatically supported. Grindstones that are rotating are then pressed against the wafer, and the wafer is also rotated while being held by a holder. In this manner, either surface of the wafer is entirely ground.
In grinding a wafer, unevenness of the wafer surfaces after the grinding often becomes a problem, because trouble such as circuit disconnection is caused at the time of formation of a semiconductor circuit for the wafer. Particularly, surface unevenness called “nanotopography” has components of 0.2 nm to 20 nm in wavelength λ, and the PV value (Peak to Valley value) of the unevenness is 0.1 μm to 0.2 μm or smaller. In recent years, a technique has been suggested for improving the flatness of a wafer by reducing the nanotopography.
For example, PCT International Publication No. WO/00/67950 discloses a method for reducing nanotopography by adjusting the relative positions of grindstones and a wafer (shift adjustment and tilt adjustment). By such a method, however, nanotopography remains, depending on the shape of the material wafer. To counter this problem, the invention disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2007-96015) has been suggested. By the method disclosed in JP-A No. 2007-96015, pockets are formed in the wafer facing surface of each hydrostatic pad member, and a fluid is supplied to the pockets so that the static pressure at each pocket can be adjusted. Accordingly, the nanotopography of the wafer can be minimized.
In the above described double-side grinding apparatus for wafers, each hydrostatic pad member forming a hydrostatic unit for hydrostatically holding a wafer through a fluid is placed at a very short distance from the wafer, and the fluid is also introduced to the back surface side of the hydrostatic padmember. In this manner, the surface accuracy of the wafer facing surface of each hydrostatic pad member can be adjusted after the hydrostatic pad members are attached, or the heat generated from the wafer facing surface of each hydrostatic pad member during the grinding can be certainly released through the pad members. Therefore, the fluid for applying static pressure is introduced to the back surfaces of the hydrostatic pad members, and the heat conduction to the components in the vicinities of the hydrostatic pad members is blocked as much as possible. FIG. 6-1 schematically shows a conventional hydrostatic pad member supporting structure. As shown in FIG. 6-1, a base member 2 made of stainless steel is placed to face the back surface of a hydrostatic pad member 1, with a void space 3 being interposed therebetween. The base member 2 and the hydrostatic pad member 1 are supported by bolts 4 having spherical washers at several spots. A fluid is also introduced to the void space 3, so as to increase the heat release effect.
However, even if pockets are formed in the wafer facing surface of each hydrostatic pad member so as to adjust the static pressure distribution, the hydrostatic pad members in practice thermally expand due to the heat generated during the grinding. As a result, the wafer facing surface of each hydrostatic pad member is deformed, and the static pressure cannot be distributed evenly onto the entire surface of the wafer. Therefore, a reduction of nanotopography cannot be expected. The cause of this problem is considered as follows. The temperature of each hydrostatic pad member rises due to the heat generated during the grinding. Since the hydrostatic pad members used in the conventional double-side grinding apparatus are made of a metal material such as aluminum or an aluminum alloy, the thermal expansion rate of each of the hydrostatic pad members is high. As a result, the spaces between the wafer and the hydrostatic pad members become uneven due to thermal expansion and contraction or slight deformation of each of the hydrostatic pad members. The unevenness of the spaces is transferred to the wafer through the fluid layers, and causes nanotopography at the time of grinding.
The hydrostatic pad member 1 has a void on its back surface side, and is attached to the metal base member 2 only by the bolts 4 having spherical washers. Therefore, its rigidity is poor, and the attachment positions shift over time. Furthermore, if there is a space between a hydrostatic pad member and a base member as in the conventional example, the fluid heated to a high temperature by the heat generated during the grinding flows into and out of the space, and the temperature of the hydrostatic pad member cannot be lowered. As a result, deformation of the hydrostatic pad member due to thermal expansion cannot be restrained.